Method and apparatus for treating a flow of gas containing oxidized sulphur compounds

ABSTRACT

A method of treating a flow of gas containing oxidized sulphur compounds, such as an exhaust gas from coal combustion, comprising the steps of (i) treating the flow of gas to obtain a first component flow containing at least 30% of the total oxidized sulphur compounds and a second component flow containing at least a part of the remainder of the total oxidized sulphur compounds; (ii) converting the first component flow into a concentrated gas containing at least 2.5% by volume of sulphur compounds; (iii) converting oxidized sulphur compounds in the second component flow into hydrogen sulphide; and (iv) supplying the concentrated gas of step (ii) and the hydrogen sulphide of step (iii) to an apparatus for manufacturing either sulfuric acid, sulphur dioxide or elemental sulphur.

TECHNICAL FIELD

The invention relates to a method and apparatus for generating sulphuricacid, condensed sulphur dioxide or elementary sulphur from a flow gascontaining oxidized sulphur compounds, such as originates for examplefrom a coal-fired electricity power plant.

BACKGROUND ART

In the operation of a coal-fired electricity plant, there is produced anexhaust gas flow which is contaminated with compounds containingsulphur, in particular oxidized sulphur compounds. In other processes,for example in the manufacture of sinters needed for charging in theblast furnace process in the iron and steel industry, waste gas flowscontaining also sulphur occur. These waste gas flows represent asignificant environmental hazard, in that their release into theatmosphere causes acidification of the environment. Consequently,production installations in which such gas flows occur are usuallyprovided with apparatus for removing the compounds containing sulphurfrom the gas flow. However, such gas cleaning systems are relativelycostly and often just shift the environmental problem from one place toanother. One known example of this is the system in which compoundscontaining sulphur are removed from a waste gas bound to chalk therebycreating a large problem of disposal.

EP-A-217567 is mentioned as describing treatment of a gas containing alarge proportion of SO₂ to prepare it for the Claus reaction whichgenerates elementary sulphur.

NL-A-7505940 describes concentration of sulphur dioxide from a waste gascontaining sulphur dioxide by absorbing the sulphur dioxide and thenregenerating a gas flow containing sulphur dioxide at higherconcentration. The latter gas flow, before being fed to a Claus reactor,is mixed with a reducing gas which is generated from the waste gas fromthe Claus reactor.

DISCLOSURE OF THE INVENTION

The object of the invention is to provide a method and apparatus forremoving the compounds containing sulphur from a gas flow while avoidingthe foregoing environmental and disposal problems, and by which valuableproducts may be obtained such as sulphuric acid or elementary sulphur.

In accordance with the invention, there is provided a method of treatinga flow of gas containing oxidized sulphur compounds to obtain one ofsulphuric acid, condensed sulphur dioxide and elementary sulphur,comprising the steps of

(i) treating said flow of gas in a wet gas scrubber to obtain therefroma first component flow from a wet gas scrubber in the form of a flow ofwater containing at least 30% of the total oxidized sulphur compounds insaid flow of gas and a second component gas flow leaving said wet gasscrubber containing at least a part of the remainder of the totaloxidized sulphur compounds in said flow of gas;

(ii) treating said flow of water in a degasifier to obtain concentratedgas containing oxidized sulphur compounds in a concentration of at least2.5% by volume;

(iii) absorbing oxidized sulphur compounds in said second component gasflow in water, and converting the oxidized sulphur compounds thereininto hydrogen sulphide; and

(iv) supplying said concentrated gas of step (ii) and said hydrogensulphide of step (iii) to a processing apparatus which manufactures fromthem one of sulphuric acid, condensed sulphur dioxide and elementarysulphur.

Typically, the second component gas flow contains, except for anyleakage losses, all the remainder of the oxidized sulphur compounds leftafter formation of the first component flow, but in the invention a partof the initial flow of gas may be used later in the process as describedbelow. Such a part is ignored in the calculation of percentage amountsin the component flow herein.

It has been found that from both the standpoint of energy consumptionand the standpoint of chemical yield, by the method in accordance withthe invention a system can be provided which is advantageous forcleaning waste gas flows containing sulphur and by which, moreover,economically valuable products may be obtained.

The invention is especially suitable for treating gas flows containingcontamination with oxidized sulphur compounds at a level below 5% byvolume, e.g. in the range 1000 to 9000 ppm.

In another aspect of the invention there is provided apparatus fortreating a flow of gas containing oxidized sulphur compounds to obtainone of sulphuric acid, condensed sulphur dioxide and elementary sulphur,comprising

(i) a first wet gas scrubber (1) for treating said flow of gas to obtaintherefrom a first component flow in the form of a first flow watercontaining at least 30% of the total oxidized sulphur compounds in saidflow of gas and a second component gas flow leaving said wet gasscrubber (1) containing substantially the remainder of the totaloxidized sulphur compounds in said flow of gas;

(ii) a degasifier (5) for treating said first flow of water to obtain aconcentrated gas containing oxidized sulphur compounds;

(iii) a second wet gas scrubber (2) for treating said second componentgas flow leaving said wet gas scrubber (1) to obtain therefrom a secondflow of water, and means (3) for converting oxidized sulphur compoundsin said second flow of water into hydrogen sulphide; and

(iv) means (6) for manufacturing one of sulphuric acid, condensedsulphur dioxide and elementary sulphur from both of said concentratedgas from said means (5) for converting said first component flow andsaid hydrogen sulphide from said means (3) for converting oxidizedsulphur compounds in said second component flow.

In the method, preferably the first component flow containing at least60% of the total oxidized sulphur compounds present in the initial gasflow, and in step (iv) the processing apparatus serves for themanufacture of sulphuric acid. It has been found to be possible in thismanner to achieve a particularly high degree of purity of sulphuric acidthus produced.

The method and apparatus according to the invention use simple means,yet attain a high concentration of the oxidized sulphur compoundspresent in the initial gas flow. For example, if the content of oxidizedsulphur compounds in the initial gas flow may vary in the range between1000 to 9000 ppm, taken on average the concentration of oxidized sulphurcompounds on leaving the degasifier may be increased by a factor of 20.

It is conventional to supply steam to a degasifier as a stripping gas,to be able to carry out the degasification. In a specific embodiment ofthe invention is it now proposed to supply air or a part of the initialflow of gas as stripping gas. This surprising solution provides a veryattractive method in terms of energy consumption, since much energy isrequired by the steam generation which is conventionally used. Moreover,this method has the advantage that the oxygen requirement in thesubsequent processing apparatus for the manufacture of the sulphurproducts may be reduced.

A water out-flow from the degasifier containing oxidized sulphurcompounds may be fed back as supply water for the wet gas scrubber. Thisachieves further optimisation of operation.

Preferably in the invention, the water in which the second component gasflow fraction is absorbed, is conveyed to an anaerobic reactor forconversion of the dissolved oxidized sulphur compounds into hydrogensulphide. This hydrogen sulphide is then mixed in the concentrated gascontaining oxidized sulphur compounds. Various systems are suitable as abiological anaerobic reactor.

The second component gas flow may comprise at least 40% of the oxidizedsulphur compounds present in the initial flow of gas. However it ispreferable for this component flow to be adjusted as small as possible,for example so that it comprises no more than 30% of the originaloxidized sulphur compounds in the flow of gas. This achieves theadvantage that the anaerobic reactor is less loaded and the associatedconsumption of additives in that reactor is reduced.

Furthermore it is desirable to supply to the anaerobic reactor a flow ofwater containing oxidized sulphur compounds originating from thedegasifier for treating the first component flow. The oxidized sulphurcompounds in this water provide an effective source of supply for theanaerobic reactor.

In a specific aspect of the invention the concentrated gas containingoxidized sulphur compounds may be mixed with the hydrogen sulphide andconveyed to a reactor for the manufacture of sulphuric acid, anapparatus for the manufacture of condensed sulphur dioxide, or to aClaus plant for the manufacture of elementary sulphur.

For the first component flow, the degasifier may be operated toadvantage at an operating pressure in the 1-1.5 bar range. This achievesthe advantage that the concentrated gas flow obtained from thedegasifier may be conveyed directly without further treatment to theabove-mentioned apparatus for the manufacture of sulphuric acid orelementary sulphur.

BRIEF DESCRIPTION OF DRAWING

The invention will now be further illustrated by way of examplereference to the drawing, which has a single FIGURE diagrammaticallyshowing apparatus embodying the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The Figure shows an apparatus in accordance with the invention and bywhich the method in accordance with the invention may be carried out. Inthe Figure a gas flow in the form of flue gas following cooling (notshown) originating from installation A is conveyed to a so-called wetgas separator or absorber 1 where the flue gas is separated into twodifferent component flows. A first component flow comprises dissolved inwater at least 30%, preferably approximately 60%, and more preferably70% of the oxidized sulphur compounds present in the flue gas. Thisfirst flow component is conveyed via a pipe 10 to a degasifier orstripper 5, in which there is obtained a highly concentrated gas flowwhich comprises at least 2.5 % vol. oxidized sulphur compounds relativeto the total gas flow which leaves the stripper 5 via the pipe 12. Thestripper 5 may be operated, for example, with steam, but preference inthe invention is given to the use of air or a part of the initial fluegas. With the gas separator 1 adjusted in such a way that the firstcomponent flow amounts to at least 30% of the initial sulphur compounds,the method in accordance with the invention is particularly suitable forthe manufacture of elementary sulphur. When the first component flowcontains 60% or more of the total initial amount of sulphur compounds,it is more appropriate to manufacture sulphuric acid. By the method inaccordance with the invention sulphuric acid may be obtained with a veryhigh degree of purity. At the same time the method in accordance withthe invention is suitable for the manufacture of condensed sulphurdioxide.

Water exiting from the degasifier 5 is conveyed back via a pipe 11 andadded to the wet gas separator 1 as supply water.

The second component flow which comprises the residue of the oxidizedsulphur compounds originally present in the flue gas is conveyed via apipe 7 from the wet gas separator 1 to a second wet gas separator 2which on the one hand provides sufficiently clean gas that is releasedinto the atmosphere through B, and on the other hand a water flow withthe oxidized sulphur compounds absorbed in water in the gas separator 2dissolved in it. This latter water flow is conveyed via a pipe 8 to ananaerobic reactor 3. At the same time the drained water from thedegasifier 5 is conveyed to the anaerobic reactor 3 via a pipe 13. Inthe anaerobic reactor 3 the oxidized sulphur compounds present in thewater convert into hydrogen sulphide (H₂ S). Then the water with thehydrogen sulphide dissolved in it leaves the reactor 3 via a pipe 9 andsludge deposited in the reactor 3 is discharged via pipe 16.

The reactor 3 is a biological anaerobic reactor, and a suitable exampleis shown in Dutch patent 166000. The dissolved SO_(x) (as sulphite orsulphate) is converted to H₂ S. A feed substrate is supplied to thereactor 3.

The pipe 9 supplies the hydrogen sulphide dissolved in water to adegasifier 4 and the drained water originating from the degasifier 4 isconveyed back via a pipe 15 to the wet gas separator 2 as supply water.Drained water which may be released without hazard to the environmentleaves the degasifier 4 via a pipe 14. The gaseous hydrogen sulphidereleased in the degasifier 4 is mixed with the concentrated gascontaining oxidized sulphur compounds from the degasifier 5 and conveyedto an apparatus 6 which is suitable for the manufacture of sulphuricacid, or alternatively, for the manufacture of elementary sulphur. Inthis latter case a Claus plant, of itself known, may be used for this.

Details of the various components of the apparatus of the Figure neednot be given here, since suitable examples are well-known to the processengineer. For example, the wet gas separators 1,2 are open spray towersoperating in countercurrent manner. Suitable strippers as the devices3,5 are well-known and effect removal of the dissolved SO_(x) from thewater into the gas phase. Acidity and temperature of the water areincreased, and then the water is contacted with the stripping gas.Sulphuric acid manufacturing plants and Claus plants suitable for theplant 6 are also well-known.

The invention is not limited to the particular embodiment shown, andvariations and modifications are possible within the concept of theinvention.

We claim:
 1. A method of treating a flow of gas containing oxidizedsulphur compounds to obtain one of sulphuric acid, condensed sulphurdioxide and elementary sulphur, comprising the steps of(i) treating saidflow of gas in a wet gas scrubber to obtain a flow of water containingat least 30% of the total oxidized sulphur compounds in said flow ofgas, and a second gas flow containing the remainder of the totaloxidized sulphur compounds in said flow of gas; (ii) treating said flowof water in a degasifier to obtain gas containing sulphur dioxide in aconcentration of at least 2.5% by volume; (iii) contacting said secondgas flow with water to absorb oxidized sulphur compounds in said secondgas flow in water, and converting the oxidized sulphur compounds thereininto hydrogen sulphide; and (iv) supplying the sulfur dioxide containinggas of step (ii) and said hydrogen sulphide of step (iii) to a processwhich manufactures from them one of sulphuric acid, condensed sulphurdioxide and elementary sulphur.
 2. The method according to claim 1,wherein said flow of water contains at least 60% of the total oxidizedsulphur compounds in said flow of gas, and in step (iv) sulphuric acidis manufactured.
 3. The method according to claim 1, including supplyingat least one of air and a part of said flow of gas to said degasifier asa stripping gas therefor.
 4. The method according to claim 1, includingsupplying a regenerate water outflow containing oxidized sulphurcompounds from said degasifier to said wet gas scrubber.
 5. The methodaccording to claim 1, comprising in step (iii) converting of theoxidized sulphur compounds absorbed in water into hydrogen sulphide inan anaerobic reactor.
 6. The method according to claim 5, includingsupplying a water outflow containing oxidized sulphur compounds fromsaid degasifier to said anaerobic reactor.
 7. The method according toclaim 1, wherein said second gas flow contains not more than 30% of thetotal oxidized sulphur compounds in said flow of gas.
 8. The methodaccording to claim 1, wherein said degasifier operates at a pressureranging from 1 to 1.5 bar.
 9. Apparatus for treating a flow of gascontaining oxidized sulphur compounds to obtain one of sulphuric acid,condensed sulphur dioxide and elementary sulphur, comprising(i) a firstwet gas scrubber (1) for treating said flow of gas to a flow of watercontaining at least 30% of the total oxidized sulphur compounds in saidflow of gas; (ii) a degasifier (5) for treating said first flow of waterto obtain a gas containing oxidized sulphur compounds; (iii) a seconddownstream wet gas scrubber (2) for receiving the gas flow from thefirst wet scrubber (1) containing the remainder of the total oxidizedsulphur compounds in said flow of gas and treating said gas flow toobtain therefrom a second flow of water containing oxidized sulfurcompounds, and converting means (3) for converting oxidized sulphurcompounds in said second flow of water into hydrogen sulphide; and (iv)means (6) for manufacturing one selected from the group selected ofsulphuric acid, condensed sulphur dioxide and elementary sulphur fromboth of the sulfur oxides containing gas from said degasifier (5) andthe hydrogen sulphide from said converting means (3).
 10. Apparatusaccording to claim 9, including means for feeding a water outflow fromsaid degasifier (5) to said first wet gas scrubber (1).
 11. Apparatusaccording to claim 10, wherein said converting means (3) for convertingsaid second flow of water comprises an anaerobic reactor for convertingoxidized sulphur compounds to hydrogen sulphide.